Although President Reagan this week has appeared to be making concessions to congressional moderates to win approval for the MX missile, he has thus far not agreed to do much more than follow the recommendations of the Scowcroft commission, whose findings he endorsed on April 19.

The president's responses this week have had the desired positive impact on Capitol Hill. They have finally put Reagan within reach of victory on the MX intercontinental ballistics missile after two years of disappointment, although that victory is not yet assured.

But Reagan's real concession actually came last month when, in endorsing the Scowcroft commission's recommendation that MX be deployed in existing missile silos, he dropped two years of rhetoric and reversed himself by agreeing that the missile could not be based in a way that would make it invulnerable to Soviet attack, and that in the future the United States perhaps should turn to smaller missiles which might be less tempting targets and less of an inducement to start a war.

Last week, however, the president was pressed by groups of 10 congressmen and three senators to go further and reaffirm his commitment to arms control by acting on three other recommendations in the Scowcroft report.

One involved modifying the U.S. proposal at the strategic arms reduction talks (START) in Geneva to focus on limiting warheads rather than missile launchers, thus bringing it into line with the recommended shift from large missiles with multiple warheads to smaller missiles with one warhead each.

The administration, however, already seemed headed in this direction. It had set up a group to work out possible changes in the proposal before the congressional letters arrived. The group has not yet made specific recommendations.

The U.S. proposal already calls for limitations of 5,000 warheads each for the United States and the Soviet Union, so the main question is whether to drop or change the companion proposals of 850 missiles per side and specific sub-ceilings of 210 for all three of the biggest Soviet missiles. Officials are not agreed on how to change these proposals, however.

The lawmakers also pressed Reagan to accept a "guaranteed build-down concept" in which two nuclear warheads would be retired for every new one deployed. The president yesterday said such an idea might be useful if it could be carefully implemented. But he specifically did not endorse a 2-for-1 trade and his advisers tell him that could be extremely troublesome.

His limited endorsement, however, appeared to be enough for the idea's two key supporters, Sens. Sam Nunn (D-Ga.) and William S. Cohen (R-Maine). Nunn said he viewed Reagan's response as a "firm commitment to the concept" and that the 2-for-1 idea was only an example and not something Reagan would be locked into. Cohen said he, too, was not seeking "a rigid application" but rather a general formula for reductions which he felt, in practice, would work out at close to 2 to 1.

Nunn's aides pointed out that the build-down also calls for any formula to be mutually negotiated with Moscow rather than unilaterally applied to U.S. forces, a provision that generally has been overlooked.

Finally, the lawmakers wanted Reagan to set up a bipartisan commission to advise him on implementing the Scowcroft report. Reagan also gave a conditional endorsement to this suggestion, saying he saw "merit" in it.

This idea has many opponents in the administration who believe organizations such as the Arms Control and Disarmament Agency and the general advisory commission on arms control can do the job. They also view such a demand as a challenge to the integrity of the administration.

But White House officials say that Reagan is committed to some such commission, despite the opposition of some advisers, in an effort to maintain the bipartisan backing he now believes is essential to achieve his major goals: deployment of the MX missile and maintenance of his arms control stand in Geneva.

White House officials claim Reagan is actually well ahead of some elements of his administration in his commitment to arms control and bipartisanship on arms issues. Nevertheless, they agree that pressure from the congressional moderates has probably speeded things up in the White House.

Most bothersome to Reagan in the recent bargaining with Congress, they say, has been the indirect challenge to his integrity in the repeated demands that he reaffirm his commitment to arms control.

Rep. Les Aspin (D-Wis.), however, sees it differently, saying the moderates have kept Reagan's feet to the fire on arms control and will continue to do so. "He hasn't given up much," he said of Reagan's responses to the congressional requests, "but we've got him on board and that is not trivial."

Aspin, one of those who wrote to the president, says he believes Reagan is committed to the Scowcroft recommendations, bipartisanship and the pursuit of an arms control agreement.